Try some of the exercises on the site below. These are fine grammatical distinctions that might or might not be taught carefully in these times. Our descriptivists here will most likely shiver...and I seriouly doubt that most of my students have been exposed to most of these distinctions:

http://www.englishpage.com/modals/can.html


Here's a direct link to a level one exercise:

http://englishpage.com/modals/interactivemodal1.htm

Edit: I'm learning a lot. Great fun tonight. I even found an Old English grammar quiz site I'll paste below. Great fun. This is true word-nerdism:

http://www.engl.virginia.edu/OE/OEA/index.html